[volt-nuts] cadmium solder alloy for low thermal emf?

John Devereux john at devereux.me.uk
Sat Jun 1 01:58:53 EDT 2013

m k <m1k3k1 at hotmail.com> writes:

>> From: john at devereux.me.uk
>> To: volt-nuts at febo.com
>> Date: Fri, 31 May 2013 14:05:39 +0100
>> Subject: Re: [volt-nuts] cadmium solder alloy for low thermal emf?
>> "Andreas Jahn" <Andreas_-_Jahn at t-online.de> writes:


>> > Within LT AN86 Cd60Sn40 is recommended for a limited temperature range
>> > of 0 to around 40 degrees.
>> > http://cds.linear.com/docs/en/application-note/an86f.pdf
>> >
>> > But: the thermal EMF is only zero against copper.
>> > Most precision integrated (hermetical) cirquits use Kovar. (39uV/K
>> > against copper)
>> > Relay contacts will be either copper berillium or another material.
>> > So in most cases a optimized solder for copper/copper connections will
>> > not be useful.
>> I have never understood why it matters anyway. The conductors being
>> soldered together end up in very good thermal contact. So there should
>> be no thermocouple generated by the solder-conductor interfaces (since
>> there is no temperature difference between the ends of the wires being
>> soldered).
>> So if you have copper-solder-copper, say, then the copper-solder
>> junction is microns away from the solder-copper junction and is
>> surrounded by metal. So surely they will be at the same temperature
>> unless there is a huge heat flow.
>> The referenced AN86 even suggests introducing balancing
>> copper-solder-copper junctions, by cutting tracks and bridging with
>> solder.
>> [...]
> But then why does it make a difference to stop the air currents around
> sensitive/accurate circuits?

Hmm, well, it must be because the connectors or IC legs are of a
different material than the wire/tracks. *Then*, each junction *is* a
thermocouple, so that if it sees a temperature different to another part
of the circuit it will generate an error. And if it sees an air current,
that temperature will fluctuate and so will the error voltage.

But adding some magic solder to the junction should not help :)

For the "balancing junctions" of AN86 the suggested (copper track -
solder bridge - copper track) junction consists of two opposing
thermocouples that oppose each other. Such a junction will "measure" the
temperature difference between the ends of the solder bridge. But this
will be miniscule and highly dependent on the geometry, the shape of the
tracks and solder blob.


John Devereux

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